This blog discusses current weather, weather prediction, climate issues, and other topics

Tuesday, December 13, 2011

Surface, Air, and Soil Temperatures: The Differences Can Be Large and Important

Meteorologists have more than one temperature reported at or near the surface and knowing the difference between them can save your live or your crops.

This blog will describe them and show you some recent examples.

First, some temperature 101! When a TV weathercaster gives the temperature at some location (Sea-Tac, Portland, Wenatchee, Forks, etc) or you get the temperature from some weather web site or app, at what height is the temperature? The answer is generally about 2 meters (roughly 6 ft), the standard height used for temperature sensors (here is the NWS temperature sensor--the white thing with the dome top):

When you get an air temperature from the sensor in your car, the height is generally about half the official height...roughly 2 feet.

Some locations measure the surface temperature---the actual temperature of the surface. Sometimes they use a sensor in a road or on the surface of the ground, other times they use infrared thermometers--like the ones used in doctor's offices to measure the temperature in your ear. Here is an example--they are called infrared radiometers:

Finally, a number of weather observing sites, particularly agriculturally oriented locations, measure the soil temperatures at one or several depths:

Now here is the interested part---these three temperatures... measured only feet apart in the vertical...can be very, very different, and the differences depend on season and time of day.

During a relatively clear, low-wind period, like we have had the past few weeks, when there is good radiational (infrared) cooling to space, the ground cools off rapidly at night. In such situations, the 2-m air temperature can be 2-8F warmer than the surface. On the other hand, temperatures in the ground are generally much warmer in such situation, and the deeper you go the warmer the subsurface gets. On the other hand, during the day, and particularly during the summer, the surface gets heated by the sun and can be MUCH warmer than either the air temperature or the ground temperature.

Not many locations have all three sensors (air temperature at 2 meters, surface temperature, ground temperature). One network that has both soil and air temperatures is the Washington State AgriWeatherNetwork, run out of WSU in Pullman (a wonderful collection of data and a great investment by the State). Here are the air temperatures (2-m) around 10 AM this morning...20s in eastern WA and low 30s in the west.

Take a look at the soil temperatures at 8 inches into the soil. MUCH warmer...low 30s in the east and mid to upper 30s in the west...even some low 40s. Why are the soils warmer? It takes time for the cold to spread into the soil.

To show this, here are the soil temperatures for the AgriWeatherSite in Woodinville over the past month. A general slow decline in temperatures over the past two weeks of cold from the mid-40s into the upper 30s. The daily heating/cooling cycle is really damped down in the soil. The farther you go down, the less daily temperature change.

What about the surface temperatures?

Here is a comparison of the surface and air temperatures at Marblemount, Washington for the past few days (times in GMT):

During the night, the surface temperatures (blue line) can be 5-7F less than the air temperatures, while during the day, the air surface and air temperatures are more similar, with the ground temperature even getting warmer for a while.

Last night I went outside with an infrared radiometer measuring the surface temperatures outside my house and comparing it to the 2-m air temperature (which was 30F)--yes, my neighbors are used to my quirky behavior. My concrete driveway was 27F and the grass was around 25F. Why was the concrete warmer? It was getting some heat conducted up from below, while the grass had some air space below to lessen upward conduction of heat.

Can this information save your life? You bet. As I have mentioned before, the greatest weather threat to your life is probably roadway icing. On cold, relatively clear nights, the road surface temperature can easily be a few degrees cooler than the 2-m air temperatures. So if reported temperatures are below 35-36F, you better be careful--the road could be at freezing or below. If the temperature sensor on your car is below 35F, better be careful.

For the gardener, don't think that you don't have to worry about protecting frost-sensitive plants if the temperatures are above freezing. Protect when temperatures are below 35-36F.

Those are some interesting points ... the thoughts of temperatures and what height the measurements are at kind of point out the lack of exposure that meteorologist get and how the meteorology world does actually work. Thanks

I was worried about an irrigation water pipe freezing, located 12" underground. Air temperatures were getting down to 20F. So I used an IR gun to check the pipe one cold morning (looking through an access point for underground valve). It was 32F, but not frozen. The big surprise: surface temperatures of grass were single digits! It was a clear night, no wind, and elevation here is 7600 ft.